In injection moulding processes, liquid materials are injected into special moulds, cooled after the injection process and subsequently removed.
In this case, the entire process is affected by different states of pressure, temperature, frictions, aggregation states and the like that are associated with the respective injection moulding material, the mould and the injection moulding method.
As a rule, the injection moulding material needs to be heated and then injected into more or less complex moulds under high pressure, e.g., up to 100 bar, wherein the liquid material flows through channels of different volumes, respectively cools or adheres to the walls, backs up at constrictions and is subjected to turns.
At the end of the initial filling process, the volume of the filling material that shrinks during the cooling process is filled with additional material by means of so-called post-injecting in order to ensure that the mould is completely filled.
The moulding tool needs to absorb the corresponding forces, ensure the appropriate heat dissipation and once again eject the injection moulding material at the end of the process.
Other system elements prepare the injection moulding material for the injection moulding process with respect to thermal and pressure-related aspects.
Due to different process variations, defects or deficiencies may respectively occur during the manufacture of the injection-moulded part.
It would be possible, for example, that                the mould is not completely filled,        a protruding element of the mould fractures,        different cooling speeds lead to tensions in the injection-moulded part,        interruptions in the cooling cycle falsify the temperature profile during the cooling process,        the injection-moulded part is damaged by defective ejectors,        the material supply or preparation is defective,        the temperature and therefore the viscosity deviates and leads to various filling and cooling defects,        explosion-like behavior under pressure leads to filling defects and burns.        
The aforementioned defects and other defects are difficult to identify. The reason for the defect needs to be determined by means of technical investigations and tests on the defective injection-moulded part. Defective injection-moulded parts frequently also cannot be readily identified as such, i.e., large quantities of rejects are produced before the defect is detected and the process can subsequently be optimized by means of experimental parameter changes.
In addition, injection moulding tools may cost several hundred thousand Euros. The monitoring of their proper function, as well as the timely initiation of maintenance procedures, therefore is of utmost economical importance.
WO2010/051954 discloses a sound-based method for checking components. However, the range of application is limited to solid components and their processing by means of machining, forming, etc.